Maybe a set of vise grip pliers??
Or small pipe wrench??
The adapter would not be used very often so using the pliers would not be too bad a solution? Assuming the pliers fit/spin within the space where the adapter connects? The pliers could also be used for other tasks that may arise, not a one tool/one job item.
There may even be a socket size that would grip the adapter. The adapter has 3 large and 3 small faces, standard nuts have 6 faces. Worth a try if you already have a socket set.
There are also different face configurations inside the socket, one type may be better than another?
See link below for one socket type that is configurable to a range of nut sizes. I've seen these but never used one.
Or this handle from etrailer